[PATCH libnvme] Implement 'dhchap_key' host and controller attributes

[Sagi Grimberg]

> Implement a 'dhchap_key' attribute for the nvme_host structure to
> support the 'dhchap_secret' connection argument, and a
> 'dhchap_key' attribute for the nvme_controller structure to support
> the 'dhchap_ctrl_secret' connection argument.

Hannes, can you add an example of a json entry to the change log?
Ideally there is some sort of documentation on what is the
format and how to populate the config file?

It is unclear to me how the user is supposed to work with it.
Is there a dump config functionality like we discussed before?